Eutrophication of lakes, rivers, and other water resources is caused by an excessive amount of dissolved nutrients in the water, such as phosphorus and nitrogen compounds. Introduced into environmental waters by untreated or ineffectively treated wastewater, these pollutants promote excessive growth of algae and other aquatic plants, which results in a decline in dissolved oxygen concentrations. In severe instances, the dissolved oxygen concentrations are reduced below the level needed to sustain fish and other animal life. Despite increased awareness of these problems and governmental action aimed at solving them, currently used methods of wastewater treatment often fall short of removing sufficient amounts of these nutrients to prevent damage to water resources. Therefore, there remains a need for an improved water treatment process to better alleviate problems caused by discharging polluting nutrients into environmental waters.
Numerous biological nutrient removal processes have been developed that, instead of chemicals, utilize microorganisms found in activated sludge to effect nutrient removal. Several of these microbiological nutrient removal processes are described in U.S. Pat. No. 3,964,998, issued Jun. 22, 1976, to Barnard and U.S. Pat. No. 5,213,681, issued May 25, 1993, to Kos, both of which are hereby expressly incorporated by reference.
The Barnard patent teaches a process for removing nitrogen from wastewater by passing the wastewater through two successive nitrogen removal stages. First, wastewater influent is mixed with return activated sludge (RAS) to form a mixed liquor prior to the first nitrogen removal stage. The RAS provides denitrifying microorganisms and additional organic material to the wastewater influent. The mixed liquor then passes through the two nitrogen removal stages, each of which in turn includes two distinct treatment zones that are categorized as anoxic or aerobic depending on their respective dissolved oxygen concentrations. In the aerobic (nitrification) treatment zone of each stage, ammonia and organic nitrogen present in the mixed liquor are converted by nitrifying microorganisms into nitrate and nitrite. In the anoxic (denitrification) treatment zone of each stage, denitrifying microorganisms reduce nitrate and nitrite in the mixed liquor to elemental nitrogen gas, N.sub.2 O gas, and other minor forms of nitrogenous gas, all of which pass from the mixed liquor into the atmosphere. In the Barnard process, mixed liquor is circulated between the anoxic and anaerobic treatment zones in the first nitrogen removal stage. After the mixed liquor passes through the dual treatment stages, it reaches a solids separation stage where it is separated into a clarified effluent and activated sludge. A portion of the activated sludge is then recycled to form the mixed liquor prior to the first stage.
The Kos patent teaches another activated sludge process that achieves nitrogen compound removal by utilizing multiple nitrogen removal stages. In each nitrogen removal stage, mixed liquor is circulated back and forth between an anoxic zone and an aerobic zone. The mixed liquor is transferred from one nitrogen removal stage to the next, either from the anoxic zone of one stage to the anoxic zone of the next stage, or from the aerobic zone of one stage to the aerobic zone of the next stage. From the final nitrogen removal stage, the mixed liquor is transferred to a clarifier where activated sludge is separated from clear effluent. A portion of the activated sludge is then recycled to the influent wastewater at a rate of approximately 100% of the influent flow rate to form the mixed liquor prior to the first nitrogen removal stage. The process may also include an anaerobic treatment zone preceding the first nitrogen removal stage for removing phosphorus from the wastewater.
While the Barnard and Kos patents disclose activated sludge processes for removing nitrogen from wastewater, ever-tightening governmental regulations dictate greater efficiency in reducing the amount of nitrogen compounds discharged from wastewater treatment facilities. In addition, existing methods of nitrogen removal currently in use typically require either deep bed nitrification filters or large anoxic tanks with external carbon sources, both of which are expensive. Accordingly, there remains a need for an improved activated sludge process that optimizes nitrogen removal from wastewater, which is cost-effective, and which minimizes capital outlays needed to retrofit existing activated sludge systems.